The "Duncan Principle" is simple, namely that the more control-I better write this down too, actually- that's a good point. The more control lost by the "Ego" the more gained by the "Id" resulting in a surprisingly predictable emotional eruption, or "breaking point". Known to Ma and Pa as a good old fashioned tantrum!

— Professor Ian Duncan

As Jeff leaves Spanish class, he bumps into Shirley. He pretends he forgot something and goes back where he runs into Britta. He explains he was trying to avoid having to walk with Shirley. Jeff says he can only stand Shirley's saccharine personality in small doses. They both head outside as Britta criticizes him for his actions and pass a group of hippies playing hacky sack. One of them, called Vaughn, greets Britta much to Jeff's surprise. Later in the cafeteria, Annie catches up to Professor Duncan and asks if she can join his latest psych lab based on his theory "The Duncan Principle". He agrees and asks that she bring two test subjects for the project.

...And there's the awkward pause.

Elsewhere in the cafeteria, Jeff is annoyed when he sees Vaughn chatting up Britta. He joins the rest of the group at a table where Pierce is showing off his latest gadget, his hearing enhancing "Ear-Noculars". Annie arrives and asks if any of them are interested in volunteering for the lab experiment. Troy agrees, but Abed says he can't because he plans on attending an Indiana Jones movie marathon at a local theater. Annie gets him to reconsider in the name of friendship and he agrees to participate. Jeff then decides to awkwardly interrupt Britta and Vaughn's conversation. Afterwards at Duncan's lab study, he explains to his students the idea behind his theory "The Duncan Principle".

Jeff interrupts Britta and Vaughn.

When put in a situation where the subject has no control, they will eventually reach a breaking point and have an emotional eruption. Duncan plans on irritate the volunteers by having them wait for the experiment to start until they react according to his theory. He sends Annie to the adjoining room to tell the volunteers to wait a few more minutes where she finds Chang amidst the volunteers. He throws a huge tantrum in response to the news and leaves. Elsewhere, Shirley and Jeff are walking back from class and trying to find common ground to relate to each other. When Shirley starts making fun of Pierce and Vaughn, the two of them start to bond.

"Houston, we have an idiot."

Back at the experiment, Duncan reviews a recording of the volunteers from the last few hours. It shows many participants proving his theory correct and becoming unhinged before leaving. The only ones left now are Troy and Abed. Troy then starts to lose it as well and crawls out of the room in tears after an emotional outburst directed at Annie. Abed is left alone in the room, and Duncan theorizes he will end up having the biggest breakdown of them all. Meanwhile, Jeff and Shirley are sitting at a bench on the quad continuing to bond by making fun of Vaughn. Their fun stops, however, after Shirley notices something and points it out to Jeff: on the lawn behind them, Britta and Vaughn are making out.

"The Soul Train Awards were tonight!"

Later, Britta finds Jeff in the student lounge seemingly devastated over the situation. She explains she wanted to tell him, but thought he would make fun of Vaughn. Jeff reassures her that as her friend, he would never do that. He goes on to say that Vaughn seems cool, and Britta suggests they all hang out sometime. The next day, Jeff catches up with Shirley as she is walking to class. When she makes a remark about Vaughn, he refuses to say anything bad about him; he wants to honor what he said to Britta and resolves to become good friends with her new beau. Back at Duncan's lab study, the professor and his students are in rough shape, frustrated that Abed still hasn't had an emotional breakdown after 26 hours. A psych student named Garrett decides to leave the study which only adds to Duncan's irritation.

Jeff is really, really, really sad.

He proceeds to have a nervous breakdown and throwing a tantrum in front of his students. He ends up angrily dismisses the class and blames Annie for having destroyed the Duncan Principle by bringing Abed. At the cafeteria, Britta, Vaughn, and Jeff are having lunch. When Britta goes to grab dessert, Shirley shows up and mocks Vaughn behind his back much to Jeff's amusement. Afterwards in the student lounge, Jeff and Britta are studying Spanish, but Britta is unable to concentrate. She confides to Jeff that she feels Vaughn is taking their relationship too seriously; as proof, she shows Jeff a poem Vaughn wrote for her. Jeff secretly takes a photo of the poem with his cell phone and later shows it off to Shirley. As Jeff and Shirley mocked Vaughn's poem, Pierce eavesdropped using his Ear-Noculars. In the study room, Annie reveals to Troy and Abed that waiting in the room until they got upset and left was the real experiment.

Jeff takes a photo of Vaughn's poem.

She's angry at Abed for not leaving, but he explains he only stayed because of their friendship. When Pierce arrives, he denounces both Jeff and Shirley having misinterpreted their prior conversation as them making fun of the study group. Jeff explains that they were ridiculing Vaughn's poem, and Shirley shows off a printout of it. As everyone derides it, Britta walks in with Vaughn. He quickly realizes they are laughing at him when he notices the photo . He assumes that Britta was in on this and leaves the room in tears. Later outside the library, Shirley apologizes to Jeff for the poem incident and admits that she has a problem with gossip. It got her kicked out of her previous social circle, but she fell back onto her bad habits because she enjoyed talking to Jeff and felt they had nothing else to share. They both agree not to gossip together anymore, but Shirley shares one last bit of gossip with Jeff: it turns out that Britta had a sex dream about him.

Shirley's gossip problems earned her the nickname "Tattlelina".

Later in the cafeteria, Annie asks Abed to forgive her, and Abed accepts her apology as well as a set of Indiana Jones DVDs she gives as a present. Elsewhere in the student lounge, Britta is less inclined to accept Jeff's apology after having been dumped by Vaughn. Jeff explains that hearing all about her boyfriend was too much for him to handle, and they agree that they don't have to share all the details of their lives. On the quad, Vaughn's bad experience with the group has left him less carefree than usual. Jeff finds another target for ridicule but sees that Shirley is now gossiping with Britta and assumes they are talking about him. He notices Pierce nearby and asks to borrow his Ear-Noculars, but Pierce has given them up because he claims "man was only meant to hear things within a natural hearing range". After Pierce walks away, Jeff surmises that the real reason was that Pierce overheard the rest of the group making fun of him.

Troy and Abed sit on the couch in the the study room, mocking everyone who walks by the window. They target Jeff as he appears, but he looks through the window and tells them that everyone outside can easily hear what they are saying.

History lesson: Shirley tells Jeff she used to have other study groups before the Spanish one but they all kicked her out for being a gossiping too much.

Discontinuity: The statue of actor and Greendale alum Luis Guzman is visible in the background during this episode even though its official unveiling does not take place until the next episode, "Advanced Criminal Law".

The DVD release has a slightly different opening from the original broadcast version. Both episodes remain virtually the same after Jeff leaves the Spanish classroom, save for two scenes involving Professor Duncan.
The version of the episode released on Amazon Prime Video uses the scenes from the original broadcast version.

Broadcast version: Chang thanks his students for filling out anonymous teacher evaluation cards. However, he pointed out one evaluation he considered too hurtful. He checked the writing style on the evaluation against old exams and figured out it was Annie. He got in her face and asked "Who is erratic and unstable now, gringa?" before kissing her on her forehead.

DVD version: Chang's latest Spanish lesson confuses the entire class who can't quite understand the things he is saying nor can they figure out the notes he quickly scrawls on the blackboard. He ends the class by saying, "Hasta luego".

The first Duncan scene is the one where Annie approaches him to ask to be a volunteer in his psych experiment. The dialogue is shortened in the DVD version as shown below:

First Duncan scene

Broadcast version:

Duncan: OK, I've actually been in this situation many times, so I'm just gonna be upfront: I'm not allowed to date students.

Annie: Oh, I wasn't...

Duncan: Even though you're an eight.

Annie (upset) : Oh!

Duncan: Which is a British ten.

Annie (smiling) : Oh.

Duncan: I'm angry. But if in spite of that, you are still interested, then you may join the lab.

Annie: (squees)

Duncan (pulls away): Ah... OK, you need to bring two human subjects. And it wouldn't hurt your chances one bit if you could lend me a fiver. I seem to have left my purse in my duffel, and my duffel in the boot of my lorry, so...

Annie (very confused) : Oh?

DVD version:

Duncan: OK, I've actually been in this situation many times, so I'm just gonna be upfront: I'm not allowed to date students.

Annie: Oh...

Duncan: But if in spite of that, you are still interested, then you may join the lab.

“You need to bring two human subjects and it would not hurt your chances one bit if you could lend me a fiver. I seem to have left my purse in my duffel and my duffel in the boot of my lorry.”— Professor Duncan

“We're just running a little bit behind, so we'll probably start in about five minutes.”— Annie

“NO!NO!NO!NONONONONO! It's unacceptable, alright! You LIED to me! When you say something starts at nine, it starts at nueve! NO!NOOOOOOOOOOOOOOOOO!”— Chang